In flat panel displays such as liquid crystals and organic EL, a high-conductive film having high transparency is much used. At present, the prevailing products are transparent films of polyethylene terephthalate (PET) or the like coated with a conductive metal oxide such as indium tin oxide (ITO) by vapor deposition or sputtering. However, for these ITO sputtered films or ion-plated films, the production equipment must be extremely large-scaled, the size of the products and the producibility are limited, and the products are expensive. What is worse, the obtained coated films are disadvantageous in that they are hard and brittle, and they have a problem of depletion and rise in the price of indium resources. Thinner and more lightweight displays of late require inexpensive, high-transparent and high-conductive films having excellent flexibility and durability.
Recently, in response to the requirements in the industrial field, conductive polymers such as polythiophene and polypyrrole have come to the front as flexible conductive materials. While improvements in conductivity of these conductive polymers and in the film formation technology have been actively pursued, a conductive polymer-containing transparent conductive film that has high conductivity and high transparency comparable to those of an ITO-sputtered film has been proposed in some quarters. However, thickness of the conductive polymer layer is limited due to the color phase peculiar to conductive polymers, and in order to avoid the coloration problem, thickness of the layer must be greatly reduced to 0.2 μm or thinner, resulting in the undeniable lack of strength of the coating film. The conductive polymer has an unsaturated bond derived from the molecular structure thereof, and has the essential drawback of deterioration by UV rays or the like, leading to a serious limitation in practical long-term use as an optical film.
On the other hand, since Nikkiso Co., Ltd. and Hyperion Catalysis International, Inc. each uniquely invented carbon nanofibrils (Patent Documents 1 and 2), a hollow carbon nanomaterial (so-called multi-walled carbon nanotubes) having a single fiber diameter of tens nm, a single fiber length of a few μm and having a crystalline graphite layer as the outermost layer has come to the front as an ultimate carbon fiber material, and development of a resin-hybridized carbon nanotube-containing composite material has been actively promoted. In 1991, Iijima et al in NEC Corporation discovered a cylindrically-formed graphene layer of a so-called single-walled carbon nanomaterial and termed it “carbon nanotube”. Since then, members in industry, government and academia who focused on the foreseeable optical and electric properties of these multi-walled and single-walled carbon nanotubes have competed in development of mass-production technology of high-purity multi-walled and single-walled carbon nanotubes and application thereof (Patent Document 3, Non-Patent Document 1).
However, since these carbon nanotubes are produced in aggregate form in which from tens to hundreds ultrafine single fibers are mutually entangled, regardless of whether the single-walled or multi-walled structure, it is extremely difficult to discretely disperse these single fibers in a solvent or a resin. This is one of the major technical obstacles to development of application of carbon nanotubes.
Recently, a method of dispersing these carbon nanotubes in water or in various organic solvents by using a dispersant such as various surfactants or polymers together with a special disperser has been disclosed (for example, Patent Document 4). However, the dispersion extremely easily reaggregates owing to the morphology and the surface property of carbon nanotubes, and is poor in storage stability. For improving the dispersibility of carbon nanotubes themselves, oxidation of the surfaces of carbon nanotube by ozone treatment or strong acid treatment has been proposed. Although this could improve the dispersibility in some degree, reduction in the important conductivity performance is pointed out. Thus, there is not any significant advantage except for special application (for example, Patent Document 5).
Further, when a dispersion liquid comprising carbon nanotubes and a dispersing solvent is directly applied onto an objective substrate, the adhesiveness thereof to the objective substrate is poor, the mechanical strength of the coating film is low, and the film is not practicable. Therefore, in general, a so-called carbon nanotube-containing coating material in which a polymer component such as resin binder is incorporated in a carbon nanotube dispersion is provided. The most widely used resin binder is a thermoplastic resin having excellent transparency, such as vinyl chloride resin and its copolymers, acrylic resin and its modified derivatives, and polyester resin and its modified derivatives (for example, Patent Document 6). For improving the film strength and the durability of thin films, a thermosetting resin such as epoxy resin, silicone resin, and isocyanate-modified urethane resin may be used as the resin binder.
However, since these resin binders and dispersants are generally electrically-insulative, they do not provide surface resistivity as low as expected to a carbon nanotube-containing coating film formed by applying a carbon nanotube-containing coating material onto an objective substrate and drying it thereon. There is an attempt to improve the conductivity of the coating film by increasing the carbon nanotube content thereof, however, surface resistivity of the resulting coating film can only be improved in some degree at the expense of transparency. As to another approach to improve surface resistivity by increasing thickness of the film, thickness of the film has its recognized limit from the viewpoint of transparency. For example, in the case where a carbon nanotube-containing coating film is designed to have a thickness of 0.5 μm and a whole light transmittance of 85%, it is thought that the surface resistivity of the coating film to be obtained is limited to around 105 Ω/square, even though the type, purity, single fiber diameter and the like of the carbon nanotubes are optimized. Accordingly, at present in the market, carbon nanotubes are recognized as transparent antistatic coating materials which are required to have a surface resistivity of from 105 to 109 Ω/square.
Further, a carbon nanotube-containing coating material which comprises a UV-curable binder resin component has been proposed as a so-called carbon nanotube-containing UV-curable coating material (for example, Patent Document 7). The electric properties and the optical properties of the coating film to be obtained are better than those of the above-mentioned carbon nanotube-containing thermoplastic or thermosetting coating materials. However, in fact, the coating film could not satisfy both high transparency (for example, a whole light transmittance of at least 85%) and high conductivity (for example, a surface resistivity of at most 105 Ω/square) required in an optical film and the like, because the UV-curable resin component is naturally electrically-insulative.
Given the current situation, various methods for improving the conductivity of carbon nanotube-containing coating films have been proposed. For example, use of a conductive polymer such as polyaniline and polythiophene as dispersant for carbon nanotubes is reported (for example, Patent Documents 8, 9, and 10). However, this cannot improve the conductivity as much as expected and has the intrinsic drawbacks attributed to conductive polymer as described above, and thereby fails to exhibit a synergistic effect of carbon nanotubes and conductive polymer.
Also hybridization of multi-walled or single-walled carbon nanotubes and conductive metal oxide fine particles has been proposed. However, this still could not exhibit a synergistic effect capable of solving the above-mentioned problems attributed to metal oxide (for example, Patent Document 11).
In a method recently proposed, a high-conductive carbon nanotube-containing coating film is proposed by so-called double-layer coating that comprises first forming a network layer of ultrathin carbon nanotubes and then infiltrating a carbon nanotube-free resin solution into the network layer (Patent Document 12). In another method proposed, carbon nanotubes are dispersively arranged on an objective substrate, then a resin film is formed on the surface of the substrate, and the formed film is separated to obtain an conductive film with carbon nanotubes embedded only in the surface part of the resin film (for example, Patent Document 13). However, these methods are still unsatisfactory for optical use in which both high transparency (for example, a whole light transmittance of at least 85%) and high conductivity (for example, a surface resistivity of less than 104 Ω/square) is indispensable, and films having sufficient strength and durability could not be produced.
In yet another method proposed, a high-conductive carbon nanotube-containing coating film is produced by applying a coating liquid that comprises carbon nanotubes and a binder resin onto an objective substrate to form a conductive layer thereon, wherein the amount of the binder resin is made smaller than the amount of the carbon nanotubes, or a carbon nanotube dispersion is applied onto a release film and dried followed by formation of an adhesive layer to obtain a transfer film which is to be transferred and fixed under pressure onto an objective substrate, thereby making the carbon nanotubes protrude out of the surface of the binder resin layer and making them electrically connected to each other (for example, Patent Document 14). However, the method has practical problems in mechanical strength and durability of the film, because the binder resin layer is too thin.    Patent Document 1: Japanese Patent 1,532,575    Patent Document 2: Japanese Patent 1,701,869    Patent Document 3: Japanese Patent 2,526,782    Patent Document 4: JP-A 2005-35810    Patent Document 5: JP-T 2000-511245    Patent Document 6: Japanese Patent 3,398,587    Patent Document 7: Japanese Patent Application No. 2006-349906    Patent Document 8: Japanese Patent 3,913,208    Patent Document 9: JP-A 2004-2621    Patent Document 10: JP-A 2004-196912    Patent Document 11: JP-A 9-115334    Patent Document 12: Japanese Patent 3,665,969    Patent Document 13: WO2006/030981A1    Patent Document 14: Japanese Patent 3,903,159    Non-Patent Document 1: SCIENCE, 306, p. 1362 (2004)